The system concerned for providing multimedia content is any support system of any service providing multimedia content online to a plurality of terminals.
The identifier concerned of a terminal of this type is an identifier accessible to this terminal. This identifier is specific to it, or specific to any other entity such as the user owning the terminal.
The multimedia content provided is audiovisual content, for example television programs, audio-only content, for example a radio program or more generally any digital content containing video and/or audio such as a computer application, a game, a slideshow, an image or any data set.
From this content, so-called temporal content will be considered more particularly below. Multimedia temporal content is multimedia content entailing a succession in time of sounds, in the case of audio temporal content, or images, in the case of video temporal content, or sound and images temporally synchronized with one another in the case of audiovisual temporal content. Multimedia temporal content may also comprise interactive temporal components synchronized with the sounds or images.
Multimedia temporal content of this type, notably when it is subject to rights such as copyright or similar rights, is typically provided in scrambled form for its protection by a Conditional Access System (CAS).
The terminology of the field of conditional access systems is thus used below in this document. The interested reader will be able, for example, to find a more complete presentation in the following document: “Functional Model of a Conditional Access System”, EBU Review, Technical European Broadcasting Union, Brussels, BE, No. 266, Dec. 21, 1995.
In particular, the terms scrambling and descrambling are therefore used for the multimedia content protected by a CAS, as synonyms for the terms encryption and decryption, which continue to be used for the other data, such as notably the control words and keys.
Multimedia temporal content is acquired by the terminal receiving on-the-fly when it is transmitted by the operator of the service.
Multimedia temporal content is then descrambled, if it was scrambled, then decoded by the terminal in order to generate a multimedia stream in unscrambled form comprising at least one temporal succession of time intervals clocked at a predefined frequency, the set of information bits necessary for displaying a single complete image or for playing a sound on a multimedia apparatus being transmitted during a single time interval, and each time interval corresponding to a single image or to a single sound, this multimedia stream being able to be played by a multimedia apparatus connected to this terminal. The specific term “audio sample” will be used below to designate the set of information bits necessary for playing a sound on a multimedia apparatus. Here, the term “in unscrambled form” indicates the fact that the multimedia stream no longer needs to be descrambled in order to be played by a multimedia apparatus in a manner directly perceptible and intelligible to a human being. The term “multimedia apparatus” furthermore designates any device capable of playing the multimedia stream in unscrambled form, such as, for example, a television set.
The terminal identification methods are particularly useful for identifying the terminal(s) used to redistribute multimedia content in unscrambled form, disregarding the rights to which they are subject, i.e. illegally. These terminals, referred to below as “pirate” terminals, are used for this purpose in a content redistribution system. These pirate terminals are identical in every respect to the other terminals used in the system for providing multimedia content and differ only in terms of the use that is made of them.
An illegal redistribution of this type, applied to audiovisual content, typically employs:
a pirate terminal which uses a service for providing multimedia content in order to acquire and decode the multimedia content provided,
a multimedia apparatus connected to this terminal in order to play the multimedia content in unscrambled form, for example a television set,
a multimedia content redistribution system comprising:                a camera to film the multimedia content as played, and generate a redistributable multimedia stream in unscrambled form, and        a redistribution server which acquires the multimedia stream filmed by the camera and transmits it via a network to a plurality of terminals.        
Users then use this redistribution system to receive, by means of a terminal, the multimedia stream in unscrambled form transmitted by this system. This redistribution system therefore allows a multitude of users to receive the multimedia stream in unscrambled form from the pirate terminal and not from the authorized operator of the content-providing service.
In such a context, it is particularly useful to identify the pirate terminal that is used in order to be able to implement countermeasures. This also allows the owner of the pirate terminal and the redistribution, or pirate, server to be traced in order to implement commercial and legal actions, for example, in respect thereof.
Methods for identifying the pirate terminal are known for this purpose. In these known methods:    a) the pirate terminal acquires then decodes multimedia temporal content in order to generate a multimedia stream in unscrambled form, and    b) a terminal identification device transmits an identification command to this pirate terminal.
In response to the identification command, the pirate terminal transmits to the multimedia apparatus, in addition to the multimedia stream in unscrambled form, an additional multimedia stream in which its identifier is encoded. In response to the reception of this additional multimedia stream, the multimedia apparatus displays the identifier of the pirate terminal, superimposed on the images of the multimedia stream in unscrambled form. The images filmed by the camera and then transmitted to the redistribution system then comprise the identifier of the pirate terminal.
In parallel, the identification device is connected to the redistribution system in such a way as to acquire the multimedia stream redistributed by this system. The identification device then acquires this redistributed multimedia stream and analyses it in order to extract the identifier of the pirate terminal from it. These known methods are referred to as “fingerprinting”.
These fingerprinting methods thus enable identification of the pirate terminal that is used. It is said that they allow the pirate to be traced.
Their first disadvantage is that these fingerprinting methods are based on the addition of an information stream supplementing the multimedia stream in unscrambled form. In terms of detectability by the pirate and robustness, this has the disadvantage that it is relatively simple for the pirate to disable them. He can, in fact, detect the additional information stream added to the multimedia stream in unscrambled form through automatic analysis of the multimedia stream that is to be redistributed, typically by means of a probe placed at the output of the terminal composer. Having detected it, he can then remove it in such a way as to re-establish the multimedia stream in unscrambled form. The redistributed multimedia stream then no longer comprises any means enabling the pirate terminal to be identified.
The identifier being clearly visible on the multimedia apparatus playing the redistributed multimedia stream, the pirate can therefore furthermore detect the additional information stream with the naked eye, i.e. more easily than through the automatic analysis mentioned above. He can then easily disable the fingerprinting method by concealing the identifier when it is displayed on the screen filmed by the camera or by then interrupting the redistribution of the multimedia stream. In terms of detectability by the pirate and robustness, the fingerprinting methods thus have an additional disadvantage.
Due to the visibility of the identifier on the multimedia apparatus playing the multimedia stream in unscrambled form, a priori disturbing for the legitimate users of the multimedia content, the fingerprinting methods furthermore have a second disadvantage in terms of impact on the experience of these users.
In order to overcome these disadvantages of the fingerprinting methods, other methods, referred to as “watermarking”, have been proposed. They consist in adding as discreetly as possible, i.e. barely detectable or undetectable to the human eye, the identifier to the multimedia stream in unscrambled form. This addition may, for example, be effected by means of small parts distributed in the images of which the succession in time constitutes the playing of the multimedia temporal content. In these watermarking methods, a multimedia apparatus receiving the multimedia stream modified in this way plays the multimedia stream in unscrambled form and simultaneously displays the identifier in parts. It therefore typically displays in a superimposed manner the video components of the multimedia stream in unscrambled form and a variable part of the identifier in such a way that the identifier is barely visible or invisible to the human eye on the multimedia apparatus.
These watermarking methods complicate the detection of the additional information stream through automatic analysis of the multimedia stream that is to be redistributed. They therefore alleviate the disadvantage of the fingerprinting methods in terms of detectability by the pirate. However, they do not eliminate this first disadvantage, and therefore remain prone to being disabled in the same way. These improvements are furthermore achieved at the expense of a new disadvantage in terms of complexity of the solution in terms of both the insertion and extraction of the identifier in the multimedia stream.
Prior art is also known from documents US 2001/054150 A1, US 2012/173342 A1, and US 2004/111740 A1.